1. Field of the Invention
An object of the present invention is fluorinated chiral liquid crystals that incorporate a cyclohexyl grouping in their structure, can be used as dopants and lead to mixtures of ferroelectric liquid crystals that have an application in display systems.
At present, only nematic liquid crystals have found application in the field of display systems. This has led chiefly to display units using the nematic helix mode and, more recently, to super-twisted nematic (STN) display units.
2. Description of the Prior Art
The drawback of display units of this type is their poor angle of view and their high response time which is of the order of about ten milliseconds. This limits their scope for development.
The invention of a display system based on the use of ferroelectric liquid crystals by N. A. Clark and S. Lagerwall (Applied Physics Letter, Vol. 36, No. 89, 1980) in 1980 has led to a new electrooptical effect induced by linear coupling between the permanent electrical polarization of the molecules and the electrical addressing field having switching times that are smaller than those of standard nematic helix displays by three orders of magnitude.
For reasons of symmetry, the ferroelectricity of the S*.sub.C phase can be observed only if the helical structure is uncoiled. This condition is achieved in the display system proposed by Clark and Lagerwell when the S*.sub.C phase is introduced into a cell with a thickness of less than 2 .mu.m.
The electrooptical performance characteristics of display units such as these depend greatly on the mesomorphic material used.
In an initial approximation, the response time .tau. of a mixture S*.sub.C is given by the following relationship: ##EQU1## where .eta. represents the rotational viscosity of the liquid crystal, P.sub.S its spontaneous polarization and E the applied electrical field.
The mixtures S*.sub.C should have a certain number of properties such as:
being thermally and photochemically stable; PA1 getting aligned easily; PA1 displaying the following phase succession: EQU S*.sub.C, S.sub.A, N*, L PA1 possessing a wide range of mesomorphic temperature PA1 having an angle of tilt close to 22.5.degree.. PA1 the succession of the phases PA1 the domain of existence in temperature of these phases, PA1 low viscosity, PA1 dielectric anisotropy and birefringence. PA1 the presence of three cycles in the chemical structure of the molecules of the formula leads to the existence of mesomorphs on a wide range of temperature; PA1 the C--F bond which provides the greatest contribution to the polarization of the mixture leads to derivatives that are more stable than similar chlorinated derivatives; PA1 finally, the use of a trans ethyl cyclohexane or cyclohexylethoxy radical plays a part in the obtaining of mixtures with low viscosity as compared with the corresponding benzoic derivatives.
To obtain an S*.sub.C mixture that fulfils all these conditions, it is preferred at present to dope an S.sub.C mixture with one or more dopants.
The S.sub.C mixture contributes:
The chiral dopants contribute the chirality and the polarization. The dopant should furthermore not modify the transition temperatures of the S.sub.C matrix and should not increase the viscosity of the final mixture.
Finally, in order to meet these conditions, the present invention proposes a new formula of fluorinated liquid crystals incorporating a cyclohexyl grouping in their structure, usable as a dopant of an S.sub.C matrix.